A spark plug used in a combustion device such as an internal combustion engine includes, for example, a center electrode extending in a direction of the axis, an insulating body provided on the outer periphery of the center electrode, a hollow cylindrical metal shell mounted on the outer side of the insulating body, and a ground electrode whose proximal end portion is joined to the leading end portion of the metal shell. The ground electrode is disposed with an approximately intermediate portion thereof bent back in such a way that the leading end portion thereof is opposed to the leading end portion of the center electrode, thereby forming a spark discharge gap between the leading end portion of the center electrode and the leading end portion of the ground electrode.
Also, a technology is known whereby a noble metal tip is provided in a region of the leading end portion of the ground electrode which forms the spark discharge gap, thus improving wear resistance and ignitability. In general, the noble metal tip is provided by an end face thereof being joined to the ground electrode by, for example, a resistance welding.
However, as the ground electrode is disposed protruding to the center side of a combustion chamber, the ground electrode is liable to become high in temperature, and the noble metal tip joined to the ground electrode is liable to become higher in temperature. Furthermore, with a recent high-powered and high-compression engine, the interior of the combustion chamber is liable to become still higher in temperature, and vibrations applied to the ground electrode and noble metal tip along with an operation of the engine are also liable to become larger. For this reason, there is fear that an oxidized scale grows quickly in a junction portion of the ground electrode and noble metal tip, and that the noble metal tip drops off from the ground electrode due to a large impact being applied along with vibrations or the like.
Therefore, a technique has been proposed whereby, in order to prevent the noble metal tip dropping off, as well as an end portion of the noble metal tip being buried into the ground electrode, the buried portion of the noble metal tip is provided with a flanged protruding portion or a constricted portion with a smaller diameter than the rest of the buried portion, thereby improving the strength of junction of the noble metal tip to the ground electrode (for example, refer to Patent Documents 1 and 2 or the like).